The United States produces more than 10 billion pounds of polypropylene annually. In a typical polymerization process, propylene monomer, catalysts and other agents are introduced into a high-pressure reactor. The raw effluent from the reactor is transferred continuously to a flash tank, from which a stream of raw polymer is withdrawn for further purification. A stream of overhead gases, containing unreacted monomer, is also withdrawn from the flash tank and is recirculated to the reactor. Thus, the propylene feed to the reactor is a combination of fresh propylene and propylene recirculated in the reactor/flash process loop. The fresh feed is usually polymer-grade propylene, a high-purity reagent that has a propylene content of 99+%, the remaining 1% or less being mostly propane, which passes unchanged through the reactor. Although the proportion of inert gas introduced into the reactor loop with the fresh feed in this way is small, the amount circulating builds up quickly, reducing the catalyst activity and reactor productivity. Propane build-up is usually controlled to a steady-state propane content in the loop in the range about 5-30%, by continuously venting a small fraction of the overhead gas from the recirculation loop.
Such a purge operation is unselective however, and, since the vent stream may contain as much as 90 vol % or more of propylene, multiple volumes of propylene may be lost from the loop for every volume of propane that is purged. Even though the volume of gas vented is only a few percent of the volume of fresh feed, the propylene lost in this way may typically amount to about 5 million lb per polymerization plant per year, which, at 20 cents per pound, has a value of $1 million.
Despite its high value, propylene recovery from the purge stream by separating it from the propane before the propane is vented is generally not cost effective. Separation of propylene from propane is difficult, because of the similar physical properties, including close boiling points (propylene, -48.degree. C. and propane -42.2.degree. C.). When high-purity polymer grade propylene is manufactured, it is separated from propane in a C.sub.3 splitter, a large cryogenic distillation column that typically contains 150 or more trays. It is clearly not practical or economic to install such equipment solely for purge treatment. Pressure swing adsorption (PSA), which can make product streams of high purity, has also been considered, but available adsorbents are not very effective, and PSA systems are also costly and energy intensive.
In summary, without the ability to recover propylene from the purge gas, there is an inevitable trade-off between controlling propane concentration in the reactor and losing propylene feedstock in the purge vent stream, by which operators of polypropylene plants are constrained. By purging a chosen percentage of the effluent light overhead gas, the operator makes what is for him, in the circumstances specific to the plant, the most acceptable compromise between the two undesirable extremes of excessive propylene loss and excessive loss of reactor efficiency.
Separation of propylene from propane by means of membranes is discussed extensively in the literature. It is well known that numerous materials and membranes exist with intrinsic selectivity for propylene over propane. These include facilitated transport membranes, polymeric membranes and inorganic membranes.
Membranes are not immediately attractive, however, for propylene recovery from vent gas, because, unlike PSA and cryogenic distillation, membranes are not able to produce a high-purity propylene permeate stream and at the same time achieve high levels of propylene recovery. The reason for this is that a membrane is not a perfectly selective barrier. If the membrane area and time in which the molecules of the gas stream are in contact with the membrane surface are very small, only a very small cut of the total feed flow will permeate. Since propylene permeates faster than propane, most of this small permeate cut will be propylene. That is, the permeate stream will have high propylene purity, and the residue stream will have a composition that is not much changed from the membrane feed composition. In other words, most of the propylene that was present in the feed gas will remain on the feed side, and will be lost when that gas is vented. Propylene recovery can be increased by increasing membrane area and contact time for the gas molecules. However, in this case propane permeation will also be increased. In other words, increasing propylene recovery also results in increasing propane recovery. Thus, little propylene will remain on the feed side to be lost by venting, but the recovered gas will be of low propylene purity.
Despite this inherent difficulty, it has been proposed to apply membrane separation to the recovery of olefins from polymerization reactor vents. U.S. Pat. No. 4,623,704 describes such a process for recovering ethylene from the reactor vent of a polyethylene plant. In this case of polyethylene manufacturing, the reactor is run at very high ethylene, very low ethane levels, so that the vent stream contains 96.5% ethylene, only 2.7% ethane and smaller amounts of methane and nitrogen. The stream is passed across a cellulose triacetate membrane that is selective for ethylene over ethane. Although the membrane selectivity is poor, the membrane produces an upgraded permeate stream, now containing 97.9% ethylene and 1.5% ethane, which is considered sufficiently free of impurities for return to the reactor, and a residue stream containing 89.9% ethylene, 8.5% ethane, which is purged from the reactor loop and used as fuel gas.
A chapter by R. D. Hughes et al., entitled "Olefin Separation by Facilitated Transport Membranes", in Recent Developments in Separation Science, N. N. Li et al. (Eds), CRC Press, 1986, discusses pilot-scale tests of a facilitated-transport membrane module at a polypropylene plant. The module was used to treat vent gas from the reactor with a view to recovering propylene. The test was a technical success for the membranes, in that the module was able to produce a permeate stream typically containing about 97-99% propylene. However, since the membrane process could not produce polymer-grade propylene, the permeate was not recirculated to the reactor, and the process was not pursued.
Thus, recovery of propylene from the propane vent stream of polypropylene manufacturing reactors has long been recognized to be desirable. It has also been recognized that the recovered propylene needs to be of comparably high purity to the fresh reactor feedstock if it is to be recirculated in the process. Although methods for separating propylene from a propylene/propane mixture exist, they are impractical for vent stream treatment, either because they are too costly, or because they cannot produce sufficiently pure polymer-grade propylene.